I've never cleaned out the sump I had in my 45g in the year that I had it up and running. But I did clean out my sump last night because I was doing a major water change to try and lower nitrates. I was also doing a major cleanup of my refugium. So since I was knee deep in it, I went ahead and siphoned out whatever loose debris I could get out of the sump, scraped off algae, rinsed the chaeto I had in the sump, cleaned out the skimmer, etc.
I foresee myself cleaning it out only as needed.